Sodium bicarbonate (NaHCO3) is a mild alkaline compound with a wide range of applications including uses in human food, animal feed, flue gas treatment, and chemical industries. World production of sodium bicarbonate in 2008 is estimated at 2.8 million tons. Most of its production derives from natural and synthetic sodium carbonate (Na2CO3). The production of sodium bicarbonate is mainly made by the carbonation of a sodium carbonate aqueous solution with gaseous CO2. The sodium carbonate aqueous solution may come from purified sodium carbonate dissolved in water, or a from a partially decarbonated slurry of crude sodium bicarbonate from the Solvay process, or from a sodium carbonate solution taken out from a sodium carbonate crystallization unit fed with solutions deriving from trona or nahcolite ores.
Sodium bicarbonate is a product with a wide range of interesting properties and a very wide range of applications from high tech ingredients for the pharma industry to the human food and animal feed, and to the use in flue gas treatment. In flue gas treatment, sodium bicarbonate is most likely among the most efficient chemicals for the removal of a wide range of pollutants (most notably the acidic ones such as HCl and sulfur oxides). Its use is limited only by the competition of less efficient but much cheaper chemicals such as lime or even limestone.
The production of sodium bicarbonate is currently almost entirely made by the carbonation of sodium carbonate. In Europe, the carbonation is usually performed in situ in the soda ash plants from CO2 coproduced during the production of soda ash (mainly the CO2 generation in the lime kilns). In the United States, the carbonation is usually made in separate plants which purchase independently the soda ash and the CO2 and combine them.
An alternative method for making sodium bicarbonate is by cooling crystallization of a liquor containing sodium bicarbonate. For example, U.S. Pat. No. 6,699,447 describes a sodium bicarbonate production from nahcolite. The method for producing sodium bicarbonate from a nahcolite deposit comprises injecting water or other aqueous solution at a temperature of at least 250° F. into the deposit, dissolving sodium bicarbonate in the hot water to form a production solution and subjecting the production solution to multiple stages of cooling crystallization. The sodium bicarbonate crystals may be dewatered and dried to form a commercial sodium bicarbonate product.
Under certain storage conditions, the sodium bicarbonate crystals can decompose into CO2, water and sodium carbonate. This typically takes place at temperatures higher than 75° C. This sort of decomposition can cause the formation of sodium carbonate on the surface of particles. U.S. Pat. No. 6,284,288 relates to the use of silica as agent for controlling the thermal degradation of alkali metal, alkaline earth metal or ammonium bicarbonate.
One principal disadvantage to the use of sodium bicarbonate is its tendency to cake either by compaction or, more importantly, by exposure to high humidity conditions. The very high ability of bicarbonate to cake becomes increasingly marked as the particle size becomes finer. This in particular presents problems during the use of the sodium bicarbonate after a more or less lengthy storage.
In addition, commercially available sodium bicarbonate products have generally poor flow characteristics due to their normal particle size distributions and crystal shapes. The addition of flow aids to sodium bicarbonate to improve its flow and anti-caking properties is known.
There is continuing interest in the development of methods for providing sodium bicarbonate powders which remain in a free-flowing state, particularly when the powders normally flow poorly and/or are susceptible to agglomeration under ambient storage conditions of temperature and relative humidity. It is therefore necessary to be able to have available a sodium bicarbonate exhibiting a better stability on storage.